U.S. patent application number 10/686780 was filed with the patent office on 2004-07-01 for tilt angle detection device and method.
This patent application is currently assigned to Pioneer Corporation. Invention is credited to Ohkubo, Akinori, Yanagisawa, Takuma.
Application Number | 20040125712 10/686780 |
Document ID | / |
Family ID | 32064275 |
Filed Date | 2004-07-01 |
United States Patent
Application |
20040125712 |
Kind Code |
A1 |
Ohkubo, Akinori ; et
al. |
July 1, 2004 |
Tilt angle detection device and method
Abstract
A tilt angle detection device and method which generates, as a
first push-pull signal, a difference between light reception
signals corresponding to two light receiving surfaces of one side
of a photodetector, which are divided along the track tangent
direction; generates, as a second push-pull signal, a difference
between the light reception signals corresponding to two light
receiving surfaces of the other side of the photodetector; and
generates a tilt signal that indicates a tilt angle defined by a
normal on the recording surface of the optical recording medium at
a position of irradiation of the laser beam and the optical axis of
the laser beam in accordance to a difference between amplitudes of
the first and second push-pull signals.
Inventors: |
Ohkubo, Akinori;
(Tsurugashima-shi, JP) ; Yanagisawa, Takuma;
(Tsurugashima-shi, JP) |
Correspondence
Address: |
MORGAN LEWIS & BOCKIUS LLP
1111 PENNSYLVANIA AVENUE NW
WASHINGTON
DC
20004
US
|
Assignee: |
Pioneer Corporation
|
Family ID: |
32064275 |
Appl. No.: |
10/686780 |
Filed: |
October 17, 2003 |
Current U.S.
Class: |
369/44.32 ;
369/53.19; G9B/7.065; G9B/7.092 |
Current CPC
Class: |
G11B 7/1369 20130101;
G11B 7/00736 20130101; G11B 7/0943 20130101; G11B 7/0956 20130101;
G11B 7/13927 20130101 |
Class at
Publication: |
369/044.32 ;
369/053.19 |
International
Class: |
G11B 007/095 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 21, 2002 |
JP |
2002-305501 |
Claims
What is claimed is:
1. A device for detecting a tilt angle of an optical recording
medium recording/reproducing device provided with an optical system
that guides a laser beam irradiated from a light source onto a
recording surface of an optical recording medium, and receives at a
light receiving portion the laser beam reflected by the recording
surface of the optical recording medium, comprising: a
photodetector provided at the light receiving portion, which has a
light receiving surface divided into at least four segments along a
track tangent direction of the optical recording medium and a
direction that is perpendicular thereto, and which outputs light
reception signals corresponding to an intensity of a laser beam
received at the four segments of light receiving surface; a first
push-pull signal generator which generates, as a first push-pull
signal, a difference between the light reception signals of said
photodetector corresponding to two light receiving surfaces of one
side of the four light receiving surfaces, which are divided in the
track tangent direction; a second push-pull signal generator which
generates, as a second push-pull signal, a difference between the
light reception signals of said photodetector corresponding to two
light receiving surfaces of the other side of the four light
receiving surfaces, which are divided in the track tangent
direction; and a tilt signal generator which generates a tilt
signal that indicates a tilt angle defined by a normal on the
recording surface of said optical recording medium at a position of
irradiation of the laser beam and the optical axis of the laser
beam in accordance to a difference between an amplitude of the
first push-pull signal and an amplitude of the second push-pull
signal.
2. The tilt angle detection device according to claim 1, wherein
said tilt signal generator includes: a first PP value detection
circuit for detecting a P-P (peak-to-peak) value of the first
push-pull signal; a second PP value detection circuit for detecting
a P-P value of the second push-pull signal; and a subtracter for
subtracting the PP value detected by the second PP value detection
circuit from the PP value detected by said first PP value detection
circuit to generate the tilt signal.
3. The tilt angle detection device according to claim 1, wherein
said tilt signal generator has an averaging circuit for averaging a
level of at least one of the push-pull signal and the tilt
signal.
4. The tilt angle detection device according to claim 2, wherein
said tilt signal generator has an automatic gain control circuit;
and wherein said automatic gain control circuit is arranged at one
of an input line of each of said first PP value detection circuit
and said second PP value detection circuit, each connection line
between said first PP value detection circuit and said subtracter
and between said second PP value detection circuit and said
subtracter, and an output line of said subtracter.
5. The tilt angle detection device according to claim 1, wherein
said tilt signal generator includes: a pattern identifying device
which identifies whether or not a track pattern at a center
position in which the laser beam is irradiated on the recording
surface of said optical recording medium is a predetermined
pattern; and a switching element which turns on to relay the tilt
signal when the predetermined pattern is identified by said pattern
identifying device.
6. The tilt angle detection device according to claim 5, wherein
the predetermined pattern is a mirror surface portion.
7. The tilt angle detection device according to claim 1, wherein
said tilt signal generator includes: a pattern identifying device
which identifies a track pattern at a center position in which the
laser beam is irradiated on the recording surface of said optical
recording medium; and an arithmetic device which calculates a final
tilt signal in response to a plurality of tilt signals when
patterns of a plurality of predetermined areas are individually
identified by said pattern identifying device.
8. The tilt angle detection device according to claim 7,
comprising: a holding device which holds the tilt signals when a
pattern that indicates the predetermined area is identified by said
pattern identifying device.
9. The tilt angle detection device according to claim 7, wherein
said arithmetic device includes: a multiplication device which
multiplies a coefficient to the tilt signal for each tilt signal
when a pattern of each of the plurality of predetermined areas is
identified by said pattern identifying device; and an adder which
adds the multiplication results of said multiplication device to
calculate the final tilt signal.
10. The tilt angle detection device according to claim 9, wherein
said arithmetic device has a storage element in which the
coefficient is stored for each of the plurality of predetermined
areas.
11. The tilt angle detection device according to claim 10, wherein
said storage element has an optical recording medium on which the
coefficient for each of the plurality of predetermined areas is
recorded as a data table.
12. A tilt angle detection method of an optical recording medium
recording/reproducing device provided with an optical system that
guides a laser beam irradiated from a light source onto a recording
surface of an optical recording medium, and receives at a light
receiving portion the laser beam reflected by the recording surface
of said optical recording medium, comprising steps of: the light
receiving portion having a light receiving surface divided into at
least four segments along a track tangent direction of the optical
recording medium and a direction that is perpendicular thereto,
outputting light reception signals corresponding to an intensity of
a laser beam received at each of the four segments of light
receiving surface; generating, as a first push-pull signal, a
difference between the light reception signals of said
photodetector corresponding to two light receiving surfaces of one
side of the four light receiving surfaces, which are divided in the
track tangent direction; generating, as a second push-pull signal,
a difference between the light reception signals of said
photodetector corresponding to two light receiving surfaces of the
other side of the four light receiving surfaces, which are divided
in the track tangent direction; and generating a tilt signal that
indicates a tilt angle defined by a normal on the recording surface
of said optical recording medium at a position of irradiation of
the laser beam and the optical axis of the laser beam in accordance
to a difference between an amplitude of the first push-pull signal
and an amplitude of the second push-pull signal.
13. An optical recording medium with which a laser beam emitted
from a light source in an optical recording medium
recording/reproducing device is irradiated on a recording surface,
and data is reproduced by receiving the reflected laser beam;
wherein a coefficient for each predetermined area of a recording
surface is recorded as a data table in order to generate a final
tilt signal by individually multiplying the coefficient to a tilt
signal that indicates a tilt angle defined by a normal at a
position of irradiation of the laser beam and the optical axis of
the laser beam, and adding the multiplication results.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a tilt angle detection
device and a method for detecting a tilt angle, which is defined as
the angle between the normal on a recording surface of an optical
recording medium at a position of irradiation of a light beam and
the optical axis of the light beam.
[0003] 2. Description of the Related Art
[0004] In order to read recorded data correctly from an optical
recording medium such as an optical disk, it is necessary to
irradiate a reading beam perpendicularly onto the recording surface
of the optical recording medium. However, when warping occurs in
the optical recording medium, or when the error in the mechanical
system is too great, the reading beam cannot be irradiated
perpendicularly onto the recording surface of the optical recording
medium, reducing the precision of data reading.
[0005] For this reason, a recorded information reproduction device
that reproduces recorded information from an optical recording
medium detects the inclination (tilt angle) that occurs between a
pickup (data reading means) and the optical recording medium with a
tilt angle detection device, and restricts reduction in the
precision of data reading by carrying out tilt correction
processing, in which the entire pickup is inclined in response to
the amount of detected tilt.
[0006] However, in the conventional tilt angle detection device,
there is a problem that a higher equipment cost are involved and
equipment becomes more complicated since a tilt detection mechanism
such as a tilt sensor is especially required to detect the tilt
that occurs between the pickup and the optical recording medium.
Furthermore, there is another problem that, at the stage of
manufacturing tilt servos, it is necessary to provide an adjustment
process in which the tilt detection mechanism is properly adjusted,
and that adjustment process is comparatively time-consuming and
intricate.
SUMMARY OF THE INVENTION
[0007] An object of the present invention is to provide a tilt
angle detection device and method that can accurately detect a tilt
angle without using a tilt detection mechanism such as a tilt
sensor.
[0008] According to the present invention, there is provided a tilt
angle detection device of an optical recording medium
recording/reproducing device provided with an optical system that
guides a laser beam irradiated from a light source onto a recording
surface of an optical recording medium, and receives at a light
receiving portion the laser beam reflected by the recording surface
of the optical recording medium, comprising: a photodetector
provided at the light receiving portion, which has a light
receiving surface divided into at least four segments along a track
tangent direction of the optical recording medium and a direction
that is perpendicular thereto, and which outputs light reception
signals corresponding to an intensity of a laser beam received at
the four segments of light receiving surface; a first push-pull
signal generator which generates, as a first push-pull signal, a
difference between the light reception signals of the photodetector
corresponding to two light receiving surfaces of one side of the
four light receiving surfaces, which are divided in the track
tangent direction; a second push-pull signal generator which
generates, as a second push-pull signal, a difference between the
light reception signals of the photodetector corresponding to two
light receiving surfaces of the other side of the four light
receiving surfaces, which are divided in the track tangent
direction; and a tilt signal generator which generates a tilt
signal that indicates a tilt angle defined by a normal on the
recording surface of the optical recording medium at a position of
irradiation of the laser beam and the optical axis of the laser
beam in accordance to a difference between an amplitude of the
first push-pull signal and an amplitude of the second push-pull
signal.
[0009] According to the present invention, there is provided a tilt
angle detection method A tilt angle detection method of an optical
recording medium recording/reproducing device provided with an
optical system that guides a laser beam irradiated from a light
source onto a recording surface of an optical recording medium, and
receives at a light receiving portion the laser beam reflected by
the recording surface of the optical recording medium, comprising
steps of: the light receiving portion having a light receiving
surface divided into at least four segments along a track tangent
direction of the optical recording medium and a direction that is
perpendicular thereto, outputting light reception signals
corresponding to an intensity of a laser beam received at each of
the four segments of light receiving surface; generating, as a
first push-pull signal, a difference between the light reception
signals of the photodetector corresponding to two light receiving
surfaces of one side of the four light receiving surfaces, which
are divided in the track tangent direction; generating, as a second
push-pull signal, a difference between the light reception signals
of the photodetector corresponding to two light receiving surfaces
of the other side of the four light receiving surfaces, which are
divided in the track tangent direction; and generating a tilt
signal that indicates a tilt angle defined by a normal on the
recording surface of the optical recording medium at a position of
irradiation of the laser beam and the optical axis of the laser
beam in accordance to a difference between an amplitude of the
first push-pull signal and an amplitude of the second push-pull
signal.
[0010] According to the present invention, there is provided an
optical recording medium with which a laser beam emitted from a
light source in an optical recording medium recording/reproducing
device is irradiated on a recording surface, and data is reproduced
by receiving the reflected laser beam; wherein a coefficient for
each predetermined area of a recording surface is recorded as a
data table in order to generate a final tilt signal by individually
multiplying the coefficient to a tilt signal that indicates a tilt
angle defined by a normal at a position of irradiation of the laser
beam and the optical axis of the laser beam, and adding the
multiplication results.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a block diagram showing a configuration of a tilt
servo control device in which a tilt angle detection device
according to the present invention is applied.
[0012] FIG. 2 is a block diagram showing a configuration of a PP
value detection circuit in the device of FIG. 1.
[0013] FIG. 3 shows the optical system of a pickup.
[0014] FIG. 4 shows a liquid crystal panel.
[0015] FIG. 5 shows a light spot formed by a laser beam irradiated
on a recording surface of an optical disk.
[0016] FIG. 6 shows amplitude waveforms of tangential push-pull
signals.
[0017] FIG. 7 shows the relationship between a radial tilt angle
and a radial tilt signal.
[0018] FIG. 8 shows another configuration of the tilt angle
detection device.
[0019] FIG. 9 shows the relationship between a frequency of pit
appearance and a radial tilt signal.
[0020] FIG. 10 shows another configuration of the tilt angle
detection device.
[0021] FIG. 11 shows another configuration of the tilt angle
detection device.
[0022] FIG. 12 shows another configuration of the tilt angle
detection device.
[0023] FIG. 13 shows the relationship between a radial tilt angle
and a radial tilt signal when there is and is not shift of an
objective lens.
[0024] FIG. 14 shows a light spot formed by a laser beam irradiated
on a mirror surface portion of a recording surface of an optical
disk.
[0025] FIG. 15 shows the relationship between a distance between
tracks and an offset component of a radial tilt signal.
[0026] FIG. 16 shows the relationship between a distance between
tracks and a radial tilt offset angle.
[0027] FIG. 17 shows another configuration of the tilt angle
detection device.
[0028] FIG. 18 and FIG. 19 each show that there is a different
pattern for each area of a recording surface of an optical
disk.
[0029] FIG. 20 shows the relationship between a radial tilt angle
and a radial tilt signal.
[0030] FIG. 21 shows another configuration of the tilt angle
detection device.
[0031] FIG. 22 shows a disk on which tables are recorded.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The following is a detailed description of embodiments of
the present invention with reference to the accompanying
drawings.
[0033] FIG. 1 shows a tilt servo control device that includes a
tilt angle detection device according to the present invention. The
tilt servo control device is applied to an optical disk player, and
has a tilt angle detection device 1, a control circuit 2, and a
liquid crystal panel 3.
[0034] As shown in FIG. 1, the tilt angle detection device 1
includes a four-segment photodetector 11, subtracters 12, 13, and
16, and PP value detection circuits 14 and 15.
[0035] The four-segment photodetector 11 is disposed in the pickup,
and has photoelectric conversion elements with light receiving
surfaces D1 to D4, which are divided into four segments in the
direction along the tangent of a data recording track of the
optical disk, and the direction of the disk radius, which is at a
right angle to the tangent of the recording track. The light
receiving surfaces D1 and D4 are positioned on the outer side of
the optical disk, and the light receiving surfaces D2 and D3 are
positioned on the inner side of the optical disk. The light
receiving surfaces D1 and D3 are in a symmetrical relationship in
regard to the center of the four segments, and also the light
receiving surfaces D2 and D4 are in a symmetrical relationship in
regard to the center of the four segments.
[0036] The photoelectric conversion elements receive light
reflected from the optical disk with the four light receiving
surfaces D1 to D4, and output electrical signals corresponding to
the intensity of received light as light reception signals S1 to
S4. The light reception signals S1 and S4 are supplied to the
subtracter 12, and the light reception signals S2 and S3 are
supplied to the subtracter 13.
[0037] The subtracter 12 subtracts the light reception signal S4
from the light reception signal S1, and supplies a tangential
push-pull signal S1-S4, which is a differential signal, to the PP
value detection circuit 14. The subtracter 13 subtracts the light
reception signal S3 from the light reception signal S2, and
supplies a tangential push-pull signal S2-S3 to the PP value
detection circuit 15.
[0038] The PP value detection circuit 14 detects positive and
negative peak values of the tangential push-pull signal S1-S4, and
calculates the P-P (peak-to-peak) value from those positive and
negative peak values. Likewise, the PP value detection circuit 15
detects the positive and negative peak values of the tangential
push-pull signal S2-S3, and calculates the P-P value from those
positive and negative peak values.
[0039] Specifically, as shown in FIG. 2, the PP value detection
circuits 14 and 15 each include a positive peak hold circuit 31, a
negative peak hold circuit 32, and a subtracter 33. The positive
peak hold circuit 31 holds the positive peak value of the
tangential push-pull signal, and the negative peak hold circuit 32
holds the negative peak value of the tangential push-pull signal.
The subtracter 33 subtracts the negative peak value from the
positive peak value, and outputs a P-P value as a signal.
[0040] The subtracter 16 subtracts the P-P value that is output
from the PP value detection circuit 15 from the P-P value that is
output from the PP value detection circuit 14, in order to generate
a radial tilt signal. The output of the subtracter 16 is the output
of the tilt angle detection device 1.
[0041] A tilt signal is supplied from the tilt angle detection
device 1 to the control circuit 2. The control circuit 2 generates
a driving signal to reduce the tilt signal. Specifically, although
not shown in the figures, the control circuit 2 is provided with a
tilt correction ROM (read only memory) in which a plurality of tilt
correction values have been pre-recorded, and reads out three
correction values that recorded at the address of the ROM specified
by that tilt signal. The read three correction values are output as
the driving signal from the control circuit 2. The three correction
values correspond to three areas 3a to 3c, which will be discussed
later, of the liquid crystal panel 3.
[0042] The liquid crystal panel 3 is arranged inside the pickup and
is capable of correcting wavefront aberration of the optical
system.
[0043] As shown in FIG. 3, in addition to the above-mentioned
four-segment photodetector 11 and the liquid crystal panel 3, the
optical system of the pickup is provided with a semiconductor laser
device 21, a collimator lens 24, a polarized beam splitter 26 with
an attached polarization plate 26a, an objective lens 27, a
focusing lens 28, and a multi-lens 29.
[0044] The semiconductor laser device 21 is driven by a driving
circuit (not shown in the figures), and emits a laser beam. The
outgoing laser beam from the semiconductor laser device 21 is
arranged to reach the polarized beam splitter 26 with the attached
polarization plate 26a via the collimator lens 24. The polarized
beam splitter 26 lets a greater portion (for example, 90%) of the
incident laser beam pass through, and the polarization plate 26a
converts the linearly-polarized light of the passed through laser
beam into circularly-polarized light.
[0045] The laser beam that has passed through the polarized beam
splitter 26 with the attached polarization plate 26a reaches the
optical disk 22 via the liquid crystal panel 3 and the objective
lens 27, and is reflected by the recording surface of the optical
disk 22. The laser beam reflected by the recording surface of the
disk 22 returns to the polarized beam splitter 26 via the objective
lens 27, the liquid crystal panel 3, and the polarization plate
26a. The polarization plate 26a converts the returning
circularly-polarized light of the laser beam that has been
reflected by the disk 22 into linearly-polarized light. The
polarized beam splitter 26 reflects the returning laser beam with a
polarization splitting surface 26b, and the reflected laser beam
arrives at the light receiving surfaces of the four-segment
photodetector 11 via the focusing lens 28, and the multi-lens
29.
[0046] The liquid crystal panel 3 is divided along the radius
direction into three areas 3a to 3c, for example, as shown in FIG.
4, an inner peripheral area, a middle area, and an outer peripheral
area. These three areas 3a to 3c are each variably controlled by
individual driving voltages output from the control circuit 2.
Therefore, the phase differences of the light that passes through
the areas 3a to 3c can be individually altered, thus making it
possible to correct wavefront aberration such as coma aberration
that occurs due to the tilt caused along the disk radius.
[0047] As shown in FIG. 5, when a light spot of a laser beam
irradiated on the recording surface of the disk 22, on which pits
(including marks) are formed, moves in the direction shown by the
arrow, variations in optical characteristics are caused in the
direction of that movement (track direction) by the intermittence
of the pits. These variations appear as signal level differences
between the light reception signals S1 and S4, and also between the
light reception signals S2 and S3 in the tilt angle detection
device 1. Therefore, the tangential push-pull signal S1-S4 obtained
by subtracting the light reception signal S4 from the light
reception signal S1, and the tangential push-pull signal S2-S3
obtained by subtracting the light reception signal S3 from the
light reception signal S2, can be detected by the subtracters 12
and 13.
[0048] As shown in FIG. 6, when there is radial tilt
(radius-direction inclination) between the disk 22 and the laser
beam irradiated onto the pits of the recording surface of the disk,
a level difference is produced between the tangential push-pull
signals S1-S4 and S2-S3. FIG. 6 shows the change of the tangential
push-pull signals S1-S4 and S2-S3 when the laser beam moves over
the pits along the track. The level differences between the signals
S1-S4 and S2-S3 become maximal at the peaks and correspond to the
size of the radial tilt. Therefore, the difference between the P-P
value of the tangential push-pull signal S1-S4 and the P-P value of
S2-S3 can be taken by the subtracter 16, and that difference is
output as a radial tilt signal that indicates the size of the
tilt.
[0049] As shown in FIG. 7, the relationship between the radial tilt
signal and the actual angle of radial tilt is approximately
proportional.
[0050] The control circuit 2 produces a driving signal so as to
reduce the size of the radial tilt in response to the tilt signals
generated by the tilt angle detection device 1, and the areas 3a to
3c of the liquid crystal panel 3 are driven in response to the
driving signal.
[0051] FIG. 8 shows another configuration of the tilt angle
detection device 1. In the tilt angle detection device 1 of FIG. 8,
an averaging circuit 17 is inserted between the PP value detection
circuit 14 and the subtracter 16, and an averaging circuit 18 is
inserted between the PP value detection circuit 15 and the
subtracter 16. The averaging circuit 17 calculates and outputs an
average value of the P-P values detected by the PP value detection
circuit 14. The averaging circuit 18 calculates and outputs an
average value of the P-P values detected by the PP value detection
circuit 15. The P-P values used in the calculations of the average
values are, for example, all the P-P values detected in a period
from the present to a predetermined time in the past.
[0052] When a pit train recorded on the disk 22 consists of pits
that appear randomly and not at a predetermined period, there are
fluctuations in the levels of the P-P values detected by the P-P
value detection circuits 14 and 15. That is, the levels of the P-P
values fluctuate in accordance with the period (frequency) with
which the pits appear. At these times, as shown in FIG. 9, the
levels of the radial tilt signal fluctuate in correspondence to the
frequency normalized to NA/.lambda.. It should be noted that NA is
the numerical aperture of the objective lens 27, and .lambda. is
the wavelength of the laser beam. Normalized frequency refers to
the frequency expressed as a ratio taking a frequency corresponding
to NA/.lambda. as 1.
[0053] In the tilt angle detection device 1 of FIG. 8, after the
average values of the P-P values are obtained by the averaging
circuits 17 and 18, the difference between the average P-P values
is calculated by the subtracter 16, so that a radial tilt signal
with reduced level fluctuation can be generated even for a disk 22
that has a pit train formed by pits that appear randomly. This
makes stabilized tilt servo control possible. Furthermore, the same
effect can be obtained when an averaging circuit is applied to the
radial tilt signal in FIG. 1.
[0054] FIG. 10 shows still another configuration of the tilt angle
detection device 1. In the tilt angle detection device 1 of FIG.
10, the output of the subtracter 16 is connected to an AGC
(automatic gain control) circuit 19. The AGC circuit 19 includes an
amplifier 35 and a comparator 36. The amplifier 35 amplifies the
radial tilt signal that is output from the subtracter 16, and
outputs it as the output signal of the tilt angle detection device
1. The comparator 36 compares the magnitude of the levels of the
radial tilt signal amplified by the amplifier 35 with a reference
signal. The reference signal is a signal with a predetermined
level, and may be set to an average level of the radial tilt
signal. The comparator 36 supplies to the amplifier 35 a signal
that expresses the comparison result. The signal that expresses the
comparison result may be a level difference, or may be a binary
signal in correspondence to the magnitudes. The amplifier 35
adjusts the amplification gain in response to the signal that
expresses the comparison result, and amplifies the radial tilt
signal by the adjusted gain.
[0055] As described above, in the tilt angle detection device 1 of
FIG. 10, even if the level of the radial tilt signal that is output
from the subtracter 16 fluctuates due to the random period of the
pits recorded on the track of disk 22, these level fluctuations can
be suppressed by the AGC circuit 19.
[0056] It should be noted that, not only can the AGC circuit be
arranged at the output stage of the tilt angle detection device 1
as shown in FIG. 10, but AGC circuits may also be arranged between
the subtracter 12 and the PP value detection circuit 14, and
between the subtracter 13 and the PP value detection circuit 15,
and AGC circuits may also be arranged between the PP value
detection circuit 14 and the subtracter 16, and between the PP
value detection circuit 15 and the subtracter 16.
[0057] FIG. 11 shows still another configuration of the tilt angle
detection device 1. In the tilt angle detection device 1 of FIG.
11, a switch 41 and a pattern identifying circuit 42 are added to
the configuration of FIG. 1. The switch 41 is connected to the
output of the subtracter 16. The pattern identifying circuit 42
identifies a pit period as a pattern. The pattern identifying
circuit 42 is provided with a comparator 43 and a memory 44.
Predetermined reference data is recorded in the memory 44. The
comparator 43 is supplied with the reference data recorded in the
memory 44 and is also supplied with demodulated data. In order to
obtain demodulated data, an adder 45 that adds the output signals
S1 to S4 of the four-segment photodetector 11 and outputs an RF
signal, and a demodulator circuit 46 that demodulates the RF signal
and outputs demodulated data, are provided as shown in FIG. 11. The
output signal of the demodulator circuit 46 is supplied to the
comparator 43.
[0058] In the tilt angle detection device 1 of FIG. 11, the
comparator 43 identifies whether or not the demodulated data
matches the reference data recorded in the memory 44. When the
demodulated data and the reference data match, the comparator 43
produces an ON signal, and when they do not match, it produces an
OFF signal. Therefore, when the demodulated data and the reference
data match, the switch 41 becomes ON, and the radial tilt signal
that is output from the subtracter 16 is then output via the switch
41 and from the tilt angle detection device 1 to the control
circuit 2. If demodulated data corresponding to a predetermined
portion of the pit train recorded on the track of the disk 22 is
stored in the memory 44 as reference data, the output radial tilt
signal of the of the tilt angle detection device 1 will not be
affected by a random period of the pit train, even if there are
level fluctuations in the radial tilt signal that is output from
the subtracter 16 due to the random period of the train recorded on
the track of the disk 22.
[0059] It should be noted that the configuration having the switch
41 and the pattern identifying circuit 42 can be arranged not only
at the output stage of the tilt angle detection device 1 as shown
in FIG. 11, but a switch and a pattern identifying circuit may also
be arranged between the subtracter 12 and the PP value detection
circuit 14, and between the subtracter 13 and the PP value
detection circuit 15, and a switch and a pattern identifying
circuit may also be arranged between the PP value detection circuit
14 and the subtracter 16, and between the PP value detection
circuit 15 and the subtracter 16.
[0060] FIG. 12 shows still another configuration of the tilt angle
detection device 1. In the tilt angle detection device 1 of FIG.
12, a switch 48 and a mirror surface portion detection circuit 49
are added to the configuration of FIG. 1. The switch 48 is
connected to the output of the subtracter 16. The mirror surface
portion detection circuit 49 detects whether or not the position at
which the pickup reads from the disk 22 is a mirror surface
portion. The mirror surface portion detection circuit 49 includes a
DC component extraction circuit 50, a memory 51, and a comparator
52. The RF signal is supplied to the DC component extraction
circuit 50. The RF signal is obtained from the above-mentioned
adder 45. The DC component extraction circuit 50 extracts the DC
component of the RF signal. A level is recorded in the memory 51
for identifying the level of the RF signal at the time of mirror
surface portion reading, and this recorded level is output from the
memory 51 as a reference level. The comparator 52 compares the DC
component extracted by the DC component extraction circuit 50 and
the reference level. When the DC component is larger than the
reference level, the comparator 52 produces an ON signal, and when
the DC component is smaller than the reference level, the
comparator 52 produces an OFF signal. The ON signal makes the
switch 48 go into an ON state, and the OFF signal makes the switch
48 go into an OFF state. When the switch 48 is in the ON state, the
radial tilt signal that is output from the subtracter 16 is output
via the switch 48, and from the tilt angle detection device 1 to
the control circuit 2.
[0061] When the disk 22 is decentered, or when the objective lens
27 of the pickup is shifted in the direction of the disk radius by
the tracking servo control, the intensity of the light received at
the light receiving surfaces D1 and D4 at the outer perimeter of
the four-segment photodetector 11, and the light receiving surfaces
D2 and D3 of the inner perimeter becomes asymmetrical. This
asymmetry is included in the radial tilt signal as an offset
component in accordance with the light receiving signals S1 and S4,
and the light receiving signals S2 and S3.
[0062] FIG. 13 shows the relationship between the radial tilt
signal and the radial tilt angle when there is no shift of the
objective lens 27 and when there is a shift of the objective lens
27. In FIG. 13, the level difference between the radial tilt
signals when the radial tilt angle is 0 degrees is the offset
component.
[0063] When the radial tilt signal includes an offset component,
the liquid crystal panel 3 is driven such that the radial tilt
signal can be reduced by the radial tilt servo control. Even when
the radial tilt signal is regulated to 0, the tilt angle is in fact
regulated to an angle that is offset by the offset component.
[0064] When the position at which the laser beam irradiates the
track of the recording surface of the disk 22 is a mirror surface
portion, a light spot 30 is formed, as shown in FIG. 14 for
example. Here the distance d between the tracks is a distance that
is normalized to .lambda./NA with the numerical aperture NA of the
objective lens 27 and the wavelength .lambda. of the laser beam.
That is, it is a distance expressed as a ratio taking the distance
between the tracks corresponding to .lambda./NA as 1. FIG. 15 shows
the results of simulating by computer the relationship in the
mirror surface portion between the distance d between the tracks
and the offset component of the radial tilt signal. As can be seen
from the relationship in FIG. 15, if the distance d is larger than
approximately 0.6, the offset component can be set to substantially
0.
[0065] Therefore, in the tilt angle detection device 1 of FIG. 12,
in the period in which the mirror surface portion detection circuit
49 is detecting that a mirror surface portion is being read in
response to the RF signal, which is the reading signal from the
disk 22, an ON signal is produced from the comparator 52, and the
switch 48 is set to ON. When the switch 48 is ON, the radial tilt
signal that is output from the subtracter 16 is output via the
switch 41, and from tilt angle detection device 1 to the control
circuit 2. This radial tilt signal is a signal that occurs during
reading of the mirror surface portion of the disk 22, and contains
almost no offset component.
[0066] The result of this is that, even when the objective lens 27
shifts in the direction of the disk radius, a correct radial tilt
signal without offset can be output.
[0067] The result of calculating with computer simulation the
fluctuation in the offset angle of the radial tilt in regard to the
distance d is shown in FIG. 16. In FIG. 16, the solid line shows
the properties of when the shift of the objective lens is 10% of
the diameter of the beam spot, and the broken line shows the
properties of when the shift of the objective lens is 6% of the
diameter of the beam spot.
[0068] Normally, if the radial tilt angle is in the range of
approximately .+-.0.2.degree., there is substantially no adverse
effect on the reading signal by the pickup. With this in mind, when
the shift of the objective lens is 10% of the diameter of the beam
spot, the distance d in FIG. 16 becomes 0.45 to 0.85. Therefore,
for a tilt servo control device that uses the tilt angle detection
device 1 of FIG. 12, if d is within the range 0.45 to 0.85 in disk
22, the radial tilt signal can be corrected without having an
adverse effect on the reading signal when the shift of the
objective lens is within 10%.
[0069] FIG. 17 shows still another configuration of the tilt angle
detection device 1. In the tilt angle detection device 1 of FIG.
17, a selector 55, a time difference correction circuit 56, and an
arithmetic circuit 57 are provided instead of the switch 41 in the
tilt angle detection device 1 of FIG. 11. The selector 55 is a
selector switch, and selectively outputs input signals from either
one of the two output terminals SO.sub.1 to SO.sub.2, depending on
the identification result of the pattern identifying circuit 42.
The time difference correction circuit 56 subjects the radial tilt
signal that is output from the selector 55 to a time correction,
and outputs it to the arithmetic circuit 57. In the time difference
correction circuit 56, hold circuits 56.sub.1 and 56.sub.2 are
connected to the output terminals SO.sub.1 and SO.sub.2. The radial
tilt signal held by each of the hold circuits 56.sub.1 and 562 is
output to the arithmetic circuit 57.
[0070] The arithmetic circuit 57 includes coefficient multipliers
57.sub.1 and 57.sub.2, and an adder 58. The coefficient multipliers
57.sub.1 and 57.sub.2 multiply coefficients K1 and K2 with the
radial tilt signals output from the time difference correction
circuit 56, and supply the multiplication result to the adder 58.
The adder 58 adds the radial tilt signals multiplied with
coefficients by the coefficient multipliers 57.sub.1 and 57.sub.2,
to output the added result to the control circuit 2 as the final
radial tilt signal.
[0071] The tilt angle detection device 1 of FIG. 17 is advantageous
when the relationship of the arrangement of pit train portions in
adjacent tracks to the position of the reading track is
asymmetrical. That is, in area A1, as shown in FIGS. 18 and 19,
there are pit train portions in the adjacent track to the outer
side, and the adjacent track to the inner side is a mirror surface
portion, but the opposite situation exists in area A2. When the
reading position of the pickup is in area A1 or area A2, an offset
component is included in the radial tilt signal that is output from
the subtracter 16. The tilt angle detection device 1 of FIG. 17 is
configured so as to reduce the effect of this offset component.
[0072] In the tilt angle detection device 1 of FIG. 17, when the
reading position of the pickup is in the above-described situation
of area A1, that fact is identified by the pattern identifying
circuit 42. In response to the output signal of the pattern
identifying circuit 42, the selector 55 causes the radial tilt
signal to be held via the output terminal SO.sub.1 as a signal RT1
in the hold circuit 56.sub.1. The held radial tilt signal RT1 is
supplied to the arithmetic circuit 57. After that, when the reading
position of the pickup has advanced to the above-described
situation of area A2, that fact is identified by the pattern
identifying circuit 42. In response to the output signal of the
pattern identifying circuit 42, the selector 55 causes the radial
tilt signal to be held via the output terminal SO.sub.2 as an RT2
in the hold circuit 56.sub.2. The held radial tilt signal RT2 is
supplied to the arithmetic circuit 57. At the arithmetic circuit
57, the coefficient K1 is multiplied with the held radial tilt
signal RT1 by the coefficient multiplier 57.sub.1, and the
coefficient K2 is multiplied with the held radial tilt signal RT2
by the coefficient multiplier 57.sub.2. As shown below, the adder
58 adds the radial tilt signals multiplied by coefficients by the
coefficient multipliers 57.sub.1 and 57.sub.2, generating the final
radial tilt signal RT.
[0073] RT=K1.multidot.RT1+K2.multidot.RT2
[0074] When the coefficients K1 and K2 are approximately equal to
the adjacent distances d1 and d2, and the pit train portion in the
adjacent track on the outer side in area A1 and the pit train
portion in the adjacent track on the inner side in area A2 are
approximately equal, then K1=K2=1/2 for example.
[0075] FIG. 20 shows the relationship between the radial tilt
signals RT1 and RT2, and the radial tilt angle of each RT. The
radial tilt signal RT is a signal in which the offset components
included in the radial tilt signals RT1 and RT2 have been
removed.
[0076] In this way, when using the tilt angle detection device 1 of
FIG. 17, a final radial tilt signal is calculated using the radial
tilt signals for areas in which the timing of reading is different,
so the offset components included in radial tilt signals can be
reduced when the relationship of the arrangement of pit train
portions in adjacent tracks to the position of the reading track is
asymmetrical.
[0077] Furthermore, the coefficients K1 and K2 are not limited to
1/2, and may be a value other than that. Further still, when the
distances d1 and d2 are not approximately equal, or when the pit
train portion in the adjacent track on the outer side in area A1
and the pit train portion in the adjacent track on the inner side
in area A2 are not approximately equal, then the offset components
included in the radial tilt signals RT1 and RT2 are different, and
therefore the coefficients K1 and K2 are accordingly set to
different values.
[0078] Furthermore, both the radial tilt signals RT1 and RT2 are
held in the time difference correction circuit 56 in the above
embodiment, but it is also possible to hold only the radial tilt
signal RT1, and for the radial tilt signal RT2 to not be held but
supplied as it is to the arithmetic circuit 57.
[0079] Furthermore, as shown in FIG. 21, addition may also be
carried out after multiplying the coefficients for radial tilt
signals of a plurality of areas. In the tilt angle detection device
1 of FIG. 21, the selector 55 selectively outputs input signals
from any one of the n+1 output terminals SO.sub.0 to SO.sub.n,
depending on the identification result of the pattern identifying
circuit 42. The time difference correction circuit 56 outputs the
radial tilt signal that is supplied from the output terminal
SO.sub.0 as it is to the arithmetic circuit 57. The hold circuits
56.sub.1 to 56.sub.n are connected to the output terminals SO.sub.1
to SO.sub.n. The radial tilt signal held by each of the hold
circuits 56.sub.1 and 56.sub.n is output to the arithmetic circuit
57. The coefficient multipliers 57.sub.0 to 57.sub.n of the
arithmetic circuit 57 multiply the coefficients K0 to Kn with the
radial tilt signals RT0 to RTn that are output from the time
difference correction circuit 56, and supply the multiplication
result to the adder 58. The adder 58 adds the radial tilt signals
multiplied by coefficients by the coefficient multipliers 57.sub.0
to 57.sub.n, to output the added result to the control circuit 2 as
the final radial tilt signal. The radial tilt signal RT may be
expressed as:
RT=K0.multidot.RT0+K.multidot.RT1+ . . . +Kn.multidot.RTn
[0080] The coefficients K0 to Kn are preset as tables in
correspondence to each of the areas of the disk 22. The tables of
set coefficients K0 to Kn can be recorded on the disk 22, and can
be read out when reproducing the data of the disk. For example, as
shown in FIG. 22, a table can be recorded on the track of the disk
22 for each area of address recording. Furthermore, the recording
of the tables can be at regular intervals or at random
intervals.
[0081] It should be noted that the configuration having the
selector 55, the time difference correction circuit 56, and the
arithmetic circuit 57 can be arranged not only at the output stage
of the tilt angle detection device 1 as shown in FIGS. 17 and 21,
but may also be arranged between the PP value detection circuit 14
and the subtracter 16, and between the PP value detection circuit
15 and the subtracter 16.
[0082] In the above embodiments, a tilt servo control device is
shown that uses a liquid crystal panel 3 as a tilt angle adjusting
means in order to compensate the radial tilt signal, but a
configuration in which an actuator is provided that mechanically
adjusts the tilt of the pickup or the objective lens with respect
to the optical disk, and in which the actuator is driven in
response to the tilt error signal, may also be used.
[0083] Furthermore, the above embodiments are described for the
case of an optical disk, but the present invention can also be
applied to other optical recording media such as an optical card.
Furthermore, the above embodiments are described for the case of
using a tilt servo control device that generates a radial tilt
signal, but in regard to the cancellation of the offset component
of the radial tilt signal, it is also possible to similarly use
cancellation of the offset component of a push-pull signal that is
brought about by the radial tilt angle.
[0084] As described above, the tilt angle can be accurately
detected without using any tilt detection mechanism in particular
such as a tilt sensor, as the present invention is provided with a
first push-pull signal producing means for producing a first
push-pull signal of the difference between the light reception
signals corresponding to two of the light receiving surfaces of one
side of the four light receiving areas that are divided along the
track tangent of the light receiving device, a second push-pull
signal producing means for producing a second push-pull signal of
the difference between the light reception signals corresponding to
two of the light receiving surfaces of the other side of the four
light receiving areas that are divided along the track tangent, and
a tilt signal generating means for generating a tilt signal that
expresses the tilt angle in response to the difference between the
amplitude of the first push-pull signal and the amplitude of the
second push-pull signal. As a result, a compact pickup can be
achieved, tilt servo control device adjustments become simple, and
moreover, the cost of tilt servo control devices can be
reduced.
[0085] This application is based on a Japanese Patent Application
No. 2002-305501 which is hereby incorporated by reference.
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